A compound reverse-imagewise releasing a photographically useful group, that is, a positive working compound is known to have various functions which are not obtainable by conventional precursors in silver halide photographic materials. Thus such compounds have been energetically investigated.
Suitable positive working compounds include the immobile compounds disclosed in U.S. Pat. Nos. 3,980,479 and 4,199,354, which were first proposed.
These compounds can release a photographic reagent upon an intramolecular nucleophilic reaction in the presence of an alkali in a reduced state thereof. On the other hand, these compounds can also reduce the releasing rate of the photographic reagent when the compound is oxidized by a redox reaction in a photographic light-sensitive material. By utilizing this property of the compound, a photographically useful group can be imagewise released. However, when using such a compound, there are many problems such as the excessive formation of fog and the reduction of discrimination caused by the discrepancy of timings of, for example, oxidation and alkaline hydrolysis since they are in a competing relation with each other.
For solving the aforesaid disadvantages of positive-working photographic reagent-releasing compounds, it was considered that the positive-working compound itself is converted into a form of the oxidized product and the photographically useful group is released therefrom upon a redox reaction with a reducing agent.
Very many positive-working compounds have been developed. For example, there are positive-working compounds releasing a photographic reagent by an intramolecular nucleophilic substitution reaction after being reduced as disclosed in U.S. Pat. Nos. 4,139,389, 4,139,379 and 4,564,577, JP-A Nos. 59-185333 and 57-84453 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and positive-working compounds releasing a photographic reagent by an intramolecular electron transfer reaction after being reduced as disclosed in U.S. Pat. No. 4,232,107, JP-A Nos. 59-101649 and 61-88257, and Research Disclosure, No. 24025, IV, (April, 1984).
Furthermore, positive-working compounds releasing a photographic reagent by utilizing a bond which is cleaved by reduction have been investigated. Examples of such compounds are the compounds utilizing the reduction cleavage of a nitrogen-sulfur bond disclosed in German Patent No. 3,008,588 and the compounds utilizing a nitrogen-nitrogen bond disclosed in U.S. Pat. No. 4,619,884. Furthermore, there are the u-nitro compounds releasing a photographic reagent by cleaving the carbon-hetero atom single bond after receiving an electron disclosed in German Patent No. 3,207,583 and compounds utilizing a reduction cleavage of a carbon-hetero atom bond, such as geminal dinitro compounds of .beta.-releasing a photographic reagent after causing the reduction cleavage of a nitrogen-nitrogen (nitro group) bond described in U.S. Pat. No. 4,609,610. Moreover, compounds utilizing the reduction cleavage of a carbon-hetero atom single bond include the nitrobenzyl compounds disclosed in U.S. Pat. No. 4,343,893.
Also, as a positive working compound further increasing the choice of the design and tolerance on production and the method of use of photographic light-sensitive materials by improving both the stability and the processing activity, the compounds described in European Patent No. 220,746A2 and Kokai Giho, No. 87-6199 have been proposed.
Each of the compounds having the functions described above has many particular advantages, but it is more preferred to increase the choice of the designing and tolerance on the production and method of use of photographic materials by improving the properties thereof and the range of suitable positive-working compounds for incorporation therein. Also, it is further desirable to provide a positive-working compound having more excellent stability in a photographic material before and after photographic processing. Moreover, it is also desirable to provide a better means for controlling the release of a photographically useful component.